1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus using the same. In particular, the present invention relates to an optical scanning device suitable for an image forming apparatus such as a laser beam printer, a digital copying machine, or a multi-function printer, which has, for example, an electrophotographic process and employs a structure in which a light beam emitted from a light source means is allowed to enter into an optical deflector within a sub scanning section at a predetermined angle, and the light beam which is deflected and reflected by the optical deflector is guided onto a surface to be scanned to record image information.
2. Related Background Art
In recent years, in an optical scanning device used for an image forming apparatus such as a laser beam printer, a digital copying machine, or a multi-function printer, in order to realize increases in scanning speed and resolution of the optical scanning device, there has been utilized an overfilled scanning optical system (hereinafter also referred to as “an OFS scanning optical system”) using a polygon mirror having small diameter thereof and including a large number of reflection surfaces (deflection surfaces) as a deflection means (for example, see JP 2001-021822).
In the overfilled scanning optical system, the number of surfaces can be increased without increasing the size of the polygon mirror. Therefore, high speed scanning can be performed while a load to a motor for driving the polygon mirror is reduced.
In order to make the entire optical scanning device compact, for example, the following manners have been used. (1) A scanning lens system is composed of a single lens. (2) A scanning lens system designed for a wide view angle is disposed near the optical deflector to reduce the external size of the scanning lens system. (3) The length of an optical path is shortened.
When the OFS scanning optical system is employed, the system is generally configured as a so-called oblique incident system that allows light from the outside of a main scanning plane (in sub scanning section) to enter the deflection surface of the optical deflector. In order to uniform the amount of light on a surface to be scanned (image plane), it is desirable to allow a light beam to enter the deflection surface from the front within a main scanning section.
Here, if an interval between the scanning lens system and the optical deflector is configured short in order to reduce the size of the optical scanning device, a necessary and sufficient oblique incident angle must be set to prevent the light beam which is deflected on the deflection surface and travels to the surface to be scanned from being blocked by elements composing an incident optical system. However, when the oblique incident angle is set to a very large value, a scanning line is curved.
Thus, it is also possible to set the oblique incident angle to a small value by lengthening a distance between the deflection surface and the optical system disposed in front of the deflection surface. However, when an optical path up to the deflection surface on which light is incident is lengthened, the size of the entire optical scanning device increases.
On the other hand, the width of lenses composing the scanning lens system in the sub scanning direction is shortened to obtain a structure in which an incident light beam on the deflection surface does not pass through the scanning lens. Therefore, the incident light beam on the deflection surface can be easily transmitted. However, in such a case, a refractive index gradient (hereinafter also referred to as “GI”) from a central portion of the scanning lens to a peripheral portion thereof during lens manufacturing is likely to influence, thereby deteriorating optical performance. Thus, the width of the scanning lens cannot be significantly shortened in the sub scanning direction.
In the case where there is a variation in environment, such as a change in ambient temperature, for example, when an optical resin lens is used for the scanning lens system, a change in magnification in the main scanning direction particularly becomes a problem. In addition, in the case of the OFS scanning optical system having a so-called double path in which light passes through the same lens twice before and after the deflection and reflection as described above, the influence of the change of magnification is large.
In actual, it is necessary to separate the scanning lens system from the optical deflector at a distance such that a predetermined width is provided in the sub scanning direction to prevent the incident light beam on the deflection surface from being blocked. This inhibits a reduction in size of the optical scanning device.